I would go with your guess, Margalit says 18 - 19 mm is standard for
750 ml bottles and a #9 is standard for them. A natural # 9 is 24
+/-0.5 mm in diameter. Synthetic diameters vary due to the density
difference of the material, a Guardian is narrower than a NomaCorq or
SupremeCorq since it's denser. 17 seems a bit much to compress it.

Jellenek has some QC info on their website related to corks.

Joe

Jack wrote:
> Hello folks -
>
> My collection of scrounged bottles has been cleaned, polished and ready
> to go.
>
> The opening sizes on the bottles vary: 17 mm, 18 mm, 18.5 mm and some 19
> mm.
>
> What size corks should I use?
>
> I'm guessing that #9 would be a tight fit in the 17 mm openings, or will
> one size in fact fit all?
>
> I've been told it's better to ask a stupid question than to male a
> stupid mistake!
>
> Thanks.